I used to have a combi boiler in my old house, which I had to clean (the installers apparently hadn't flushed the system and it had seized up the pump and the maintainers wouldn't carry on the maintenance until I had it cleaned), so I bought the cleaner and inhibitor from B&Q and did as instructed and all was great.

However, I've now moved to a place which has a gas boiler (not a combi) which as far as I'm aware has a header tank. I haven't seen any kind of drain tap for it like my old combi did. When I bled the rads, the water was pitch black and the radiators gurgle when they are on. I'm guessing it could do with a good clean out and the system drained, possibly with some sludge cleaner put in. I know it's well over 20 years old and I can bet it hasn't ever been cleaned.

So, first of all, is this a good idea or am I likely to screw the whole system up, and secondly, how do I drain the system?

But if he can't find a drain cock, how is he going to flush/clean the system out ?

So I'm basically looking for a tap type valve thing somewhere downstairs, or under the floor?

Should I turn off the water supply to the header tank in the loft so that the system doesn't keep filling up, and then once I've put the cleaner in etc let the water back in?